1. Field of the Invention
This invention relates generally to the actuation of electromagnetic clutches and more particularly to a controller for such clutches that reduces the stresses associated with engagement of the clutches by providing a progressive or soft start.
2. Description of Related Art
Electromagnetic clutches are used in a variety of applications, including coupling large and small engines and motors to equipment operated by the engines or motors. Especially in the case of relatively small engines and motors, the price of clutch controllers is a significant factor in the implementation of such controllers. However, small engine applications also benefit significantly from controlling the abrupt engagement of clutches since such engagement may increase wear, resulting in undesirable operating characteristics such as jerking, or cause the engine to stall if the clutch is engaged abruptly.
There have, in the past, been some efforts made towards reducing the abruptness of clutch engagement. Such methods have taken various forms, including mechanical arrangements that suffer from the disadvantage that they are complex and expensive, and electrical arrangements that have provided less than optimal results. Since an electromagnetic, clutch requires a clutch controller for controlling power applied to the clutch, it would be desirable to combine such controller with method and apparatus for providing for gradual engagement of the clutch in a single unit. This invention provides such method and device.
In almost all instances, an electromagnetic clutch includes a coil or solenoid through which a current is passed to actuate the clutch and an at least partially ferrous core that is arranged to be drawn into the coil when current is supplied to the coil. The core is mechanically connected to the clutch mechanism so that when power is applied to the coil and the core is drawn into the coil, the clutch mechanism is moved from a disengaged to an engaged position. This invention controls the actuation of the clutch by controlling the current passing through the coil to provide for a gradual engagement of the clutch rather than an abrupt engagement. This invention relies on the characteristic of a solenoid type of clutch actuator that the inductance of a solenoid increases as the core is drawn into the body of the solenoid. Since, the core is mechanically connected to the clutch, movement of the core is directly related to the position and therefore the state of the clutch and by taking advantage of this, the present invention permits the position of the clutch to be determined from the increase in the inductance of the coil that occurs as the core is drawn into the coil.
Because the current flowing through the coil will tend to increase with time, according to a well-known relationship, the actual current through a coil as a function of time can be predicted relatively accurately. Where the inductance of the coil increases quickly enough as the core moves into the coil, the current through the coil will decrease rather than increase as a function of time, and by monitoring the current through the coil and recognizing this decrease in current as the clutch begins to engage, the present invention provides a method and apparatus for controlling the engagement of the clutch to provide a soft start.
It is desirable to provide a clutch controller that automatically adjusts for different clutch models. Clutches come in many different sizes, larger clutches requiring more current than smaller clutches, in prior art controllers, predetermined absolute current set points have been used to control the operation of the clutches. For example, a controller might initiate a ramp at a starting point of 1.2 amps for three amp clutch, and a starting point of 2 amps for a 5 amp clutch.
As clutches wear, more current is required to activate them. Consequently, if a fixed current is employed at the beginning of the ramp portion of the clutch activation, the clutch may disengage. Since absolute current set points always produce the same ramp current profile regardless of wear, controllers using this technique may be unreliable.
Another problem of known controllers is that the current ramp increases the current slowly from a preset value to 100%. In practice, the clutch is fully engaged at a value somewhat less than 100% and continuing the ramp past this value may cause clutch slippage and overheating.
A still further problem associated with known clutches is that clutches do not always engage squarely, especially if they are worn. If a clutch pulls in obliquely, a current sensor may indicate a false engagement when one portion of the clutch plate touches the opposite clutch plate.
While a variety of methods for controlling the current passing through the clutch may suggest themselves to those skilled in the art, and in accordance with the invention, it is preferred to control the current through the use of a pulse width modulator which can be adjusted to provide a controlled amount of current to the coil of the clutch and thereby to accomplish a soft start.